1. Field of the Invention
The present invention relates to a radiation image capturing apparatus for acquiring radiation image information of a subject, and a method of controlling such a radiation image capturing apparatus.
2. Description of the Related Art
In the medical field, there have widely been used radiation image capturing apparatus which apply a radiation emitted from a radiation source to a subject (patient), detect the radiation that has passed through the subject with a radiation detector, and record radiation image information based on the detected radiation.
The radiation image capturing apparatus of the type described above are required to achieve a good level of radiation image quality while minimizing the radiation dose applied to the subject. In order to acquire appropriate radiation image information of a region of interest (ROI) of the subject, it is necessary to establish an exposure control condition for applying a desired dose of radiation to the region of interest. There has been proposed a radiation image capturing apparatus including an AEC (Automatic Exposure Control) system for controlling a radiation dose emitted from the radiation source based on a detected radiation dose that has passed through the subject.
One known radiation image capturing apparatus is a mammographic system used for breast cancer screening. The mammographic system comprises an image capturing base housing a panel-like solid-state detector for supporting a subject's breast to be imaged, a compression plate disposed in confronting relation to the image capturing base for compressing the breast against the image capturing base, and a radiation source for applying a radiation through the compression plate to the breast.
The subject's breast comprises a mammary gland region and a fat region. In terms of breast cancer screening, the region of interest is the mammary gland region because it is highly susceptible to breast cancer. While the mammary gland region has a large coefficient of absorption of radiations, the fat region has a small coefficient of absorption of radiations and absorbs almost no radiations. In order to acquire appropriate radiation image information of the mammary gland region, it is necessary to set suitable exposure control conditions based on the mammary gland region for applying a desired dose of radiation to the mammary gland region. The exposure control conditions include a tube voltage, a tube current, a radiation exposure time, etc. to be established for the radiation source. Of these exposure control conditions, the tube current and the radiation exposure time are the most important conditions for determining a dose of radiation to be applied to the subject.
Some mammographic systems include a radiation dose information detector for AEC housed in the image capturing base, the radiation dose information detector being used such that a small output range thereof is identified as representing the region of a high mammary gland density (see Japanese laid-open patent publication No. 2000-197624 and Japanese laid-open patent publication No. 7-153592).
When a medio-lateral oblique (MLO) image, for example, is to be taken by the mammographic system, since the radiation is applied obliquely to the subject, the breast muscle comes into the radiation detection area. Generally, the breast muscle tends to have a greater coefficient of absorption of radiations than that of the mammary gland region.
Therefore, the breast muscle is represented by a lower output range of the radiation dose information detector than the mammary gland region is. As a result, the breast muscle may possibly be identified as the region of interest when the region of a high mammary gland density needs to be identified as the region of interest. If the breast muscle is identified as the region of interest, then it is difficult to obtain appropriate radiation image information of the mammary gland region, possibly making it impossible to generate a desired radiation image of the breast.